It is well known that cholesteric (also referred as chiral nematic) liquid crystal displays have optically distinct states: a planar state that reflects light, a focal conic state, and a homeotropic state that appears black if a black layer is painted on one side of the display. In the following, the planar state is also referred as the on-state, and the focal conic state as the off-state. Both the planar and the focal conic states are stable at zero voltage. The homeotropic state can only be maintained with a voltage applied across the display. Thus using the planar and focal conic states, cholesteric liquid crystal displays can be advantageously addressed by a passive matrix for many applications.
However, passive matrix addressed cholesteric liquid crystal displays have problems such as cross-talk, long frame times, and black bar artifacts. These problems can be overcome by driving the displays with an active matrix drive scheme at the higher expense of the active matrix. With the intense research and development of amorphous and poly silicon thin film transistors, active matrix drive appears to be affordable for use in high performance cholesteric liquid crystal displays. Organic thin film transistors fabricated on plastic substrates offer higher voltage outputs (e.g. 100 volts or more) that can be used to drive liquid crystal displays that require a high drive voltage.
A typical active matrix pixel drive is shown in FIG. 1 and includes a matrix of data 10 and select 12 lines. Data and select lines are also called column and row lines, respectively. An array of pixels 14 are connected to the data and select lines through active switching elements that in one example include a transistor 16 and a storage capacitor 18. The active matrix addressed liquid crystal display further includes a common electrode 20 connected to all of the pixels.
Nahm et al., Amorphous Silicon Thin-Film Transistor Active-Matrix Reflective Cholesteric Liquid Crystal Display, Proceedings of the 18th International Display Research Conference, pp. 979-982, 1998, and Kawata et al., A High Reflective LCD with Double Cholesteric Liquid Crystal Layers, Proceedings of the 17th International Display Research Conference, pp. 246-249, 1997, proposed active matrix addressed bistable cholesteric liquid crystal displays that were operated between the planar and homeotropic states.
US 2001/050666 A1 issued Dec. 13, 2001 to Huang et al. discloses active matrix addressed bistable cholesteric liquid crystal displays that made better use of the bistability of the cholesteric liquid crystal display and were operated between the planar and focal conic states. They propose driving the active matrix addressed bistable cholesteric liquid crystal displays by a multiple level voltage driver that supplies two voltage levels (+40 volts, −40 volts) to achieve the planar state, and another two voltage levels (+30 volts, −30 volts) to obtain the focal conic state.
As shown in FIG. 2, Huang et al. employ row, column, backplane, and pixel voltage waveforms varying with time t in two consecutive frames. Without loss of generality, two row voltage waveforms (also referred as select voltage waveforms) Vrow1, Vrow2 and two column voltage waveforms (also called data voltage waveforms) Vcol1, Vcol2 are used to illustrate the idea. In the first frame 30, a select voltage pulse 200 is sequentially applied to row 1 and row 2. In the meantime, data voltage waveforms Vcol1, Vcol2 are applied to column 1 and column 2. Note that the data voltage waveforms take different amplitudes in order to achieve distinct optical states. In particular, a voltage pulse of 40 volts is applied to obtain a planar state, and a pulse of 30 volts to obtain a focal conic state. The backplane is connected to zero voltage. The row voltage for selection is preferably about 5 V and, most preferably, at least 5 V higher than column voltage, in order to open or close the transistor 16. The pixel voltage VP11 formed at the intersection of row 1 and column 1 thus is 40 volts, and the pixel voltage VP22 formed at the intersection of row 2 and column 2 is 30 volts.
In the second frame 32, the backplane is set at 40 volts. The data voltage is zero for the planar state, and 10 volts for the focal conic state. The pixel voltage is then either −40 volts for VP11, 0 volts for VP12, or −30 volts for VP22. The zero pixel voltage VP12 keeps the state of the pixel unchanged.
Overall, in the prior art active matrix addressed cholesteric liquid crystal displays, more than 2 different voltage levels in addition to a zero level are required to apply to data voltage waveforms and pixel voltage waveforms. This complexity hinders the use of the active matrix to address cholesteric liquid crystal displays.
It is well known that fewer voltage level drivers would result in a lower cost. A two level voltage driver has been utilized for a passive matrix cholesteric liquid crystal display, such as disclosed by Rybalochka et al., Dynamic Drive Scheme for Fast Addressing of Cholesteric Displays, SID 2000, pp. 818-821 and Simple Drive Scheme for Bistable Cholesteric LCDs, SID 2001, pp. 882-885. They proposed U/√{square root over (2)} and U/√{square root over (3/2)} dynamic driving schemes requiring only 2-level column and row drivers, which output either U or 0 voltage, to generate a 3-level pixel voltage including +U, −U, and 0. The passive matrix 3-level drive schemes employ multiple phases, including preparation, holding, selection, and evolution phases. Since active matrix displays do not employ multiple phases as discussed in the above cited papers, it is not apparent whether or how a 3-level drive scheme could be used with an active matrix to obtain the inherent advantages of a 3-level drive scheme.
Therefore, there is a need for an improved drive scheme for an active matrix addressed cholesteric liquid crystal display having fewer voltage levels to reduce complexity of the drive scheme while achieving high optical performance.